Plasma processing systems have long been employed to process substrates into finished electronic products for creating integrated circuits (ICs). Plasmas may be generated using a variety of plasma generation technologies, including for example inductively coupled plasma, capacitively coupled plasma, microwave, electron-cyclotron resonance (ECR), etc.
During the processing of a substrate, it is highly desirable to accurately and timely monitor various process parameters inside the plasma processing chamber. Plasma-facing probe or sensor techniques, which involve exposing a surface of the probe or sensor made of an electrically conductive material to the plasma, has long been employed for such monitoring tasks. One type of plasma-facing monitoring probe that has been employed to measure the process parameters is an ion flux probe, such as that described in U.S. Pat. No. 7,319,316, entitled “Apparatus For Measuring A Set Of Electrical Characteristics In A Plasma.” In the aforementioned U.S. Pat. No. 7,319,316, a substantially co-planar probe is employed to measure the ion flux within the plasma processing chamber. The measured ion flux may then be employed for ascertaining, for example, the endpoint of a chamber conditioning process, for measuring plasma properties (e.g., ion saturation current, electron temperature, floating potential, etc.), for chamber matching (e.g., looking for differences between chambers which should nominally be identical), for detecting faults and problems in the chamber, etc.
Some production versions of the ion flux probe have been implemented in the field, and it has been discovered that opportunities for improvement are possible. To facilitate discussion, FIG. 1 shows a typical ion flux probe installation. In FIG. 1, ion flux probe 102 is disposed in an aperture 104 in an upper electrode of a plasma processing chamber. The upper electrode is typically formed of aluminum or graphite, with a plasma-facing surface 106 formed of a suitable material, such as silicon.
Ion flux probe 102 comprises a stem 110 for coupling with a support structure (of which a portion 112 is shown). Stem 110 is typically formed of an electrically and thermally conductive material, such as aluminum. An insulating ring 114 surrounds stem 110 as shown and is designed to provide centering support for stem 110 within aperture 104 as well as electrically insulate stem 110 from the rest of the upper electrode.
Ion flux probe 102 also includes a plasma-facing probe head 120, which is formed of a material that is designed to be chemically and electrically substantially similar to the plasma-facing surface 106 of the upper electrode in order to facilitate accurate measurement of parameters from the plasma (disposed below the upper electrode in the example of FIG. 1). In the case of FIG. 1, probe head 120 is formed of silicon as well. An O-ring 130 is provided to prevent contaminants from falling into the chamber through the gap 136 between ion flux probe 102 and aperture 104. Gap 136 exists due to mechanical tolerance and also to accommodate thermal expansion during the process cycles. O-ring 130 is typically formed of a resilient elastomer, and also functions to seal the process gas inside the chamber from escaping upward through the aforementioned gap.
A ring 132 is shown disposed around probe head 120. Ring 132 may be made of quartz (as in the case of the example of FIG. 1) or of another suitable dielectric material. Quartz ring 132 is designed to electrically insulate probe head 120 from the rest of the upper electrode. A secondary function of quartz ring 132 is to protect o-ring 130 from being unduly attacked by the higher energy ions and radicals of the plasma generated within the chamber.
However, it has been noted that there are opportunities for improvement in the design of the ion flux probe and in the mounting of the ion flux probe in the chamber. For example, the presence of quartz ring 132 has been found to create a chemical loading condition in the chamber during plasma processing since quartz ring 132 is a different material than the silicon material of the probe head 120 or the silicon material of the plasma-facing surface 106 of the upper electrode. During certain etch processes, such as during a dielectric etch, the etching of quartz ring 132 may change the chemical or plasma composition inside the chamber, leading to undesirable etch results on the substrate. Furthermore, as quartz ring 132 is consumed, a recess may be formed between the lower surface of the upper electrode and the plasma-facing surface of probe head 120 and may create a “polymer trap,” potentially increasing the possibility of particulate contamination on the substrate during subsequent processing cycles. Furthermore, as quartz ring 132 is eroded, measurements by the ion flux probe may be distorted since the probe head geometry as presented to the plasma has changed.
As seen in FIG. 1, a direct line-of-sight exists between tile plasma (which is formed below the upper electrode of FIG. 1) and o-ring 130. This direct line-of-sight permits plasma constituent components, such as the high energy ions and radicals, to reach the o-ring, thereby contributing to an accelerated rate of o-ring degradation. The accelerated degradation of o-ring 130 necessitates a higher frequency of maintenance for the purpose of replacing o-ring 130, which leads to more system downtime, reduced plasma system throughput, and a higher cost of ownership for the plasma processing tool.
Another issue with the arrangement of FIG. 1 pertains to the lack of mechanical referencing between ion flux probe 102 and the rest of the upper electrode. Since ion flux probe 102 is coupled to a support structure 112 that is mechanically independent of the upper electrode, it has been found that the accurately positioning ion flux probe 102 during installation to ensure that the lower surface of probe head 120 is flushed with the lower surface 106 of the upper electrode has been a challenge.
Another aspect of the ion flux probe arrangement of FIG. 1 that may also be improved relates to thermal equilibrium. For accurate measurement, it is desirable that the ion flux probe, and in particular ion probe head 120, be at thermal equilibrium as quickly as possible with the rest of the upper electrode. However, since ion flux probe 102 of FIG. 1 is mechanical coupled to support structure 112 and only contacts the remainder of the upper electrode incidentally through insulating ring 114 and quartz ring 132 (both of which are relatively poor thermal conductors), the goal of achieving fast local thermal equilibrium between probe head 120 and the upper electrode has not always been satisfactorily attained.